Chapter 2 – Efficiency of movement biomechanics
Biomechanics: the study of sports science that applies the law of mechanics and
physics to human performance. It aides in understanding performance in sports &
physical activity.
Eg. benefits injury prevention, re design of equipment and improvement in
performance through analysis.
How it works: record, observe movements, on video replay for example. Analysis
occurs, selected aspects broken down, from there quantitative and qualitative
information is gathered about the performance or skill.
What are the benefits?: Optimise sports performance by developing most efficient
technique.
Helps to prevent or reduce injuries.
Aides in re-design, or further develop improved equipment. Also benefits varying age
groups and abilities with modified equipment. Eg. kanga cricket bat.
Adaption of equipment / tools for training drills and skills eg. batting tees, ball
throwing machine.
Selection key biomechanical principals
Force production
Application of force including concepts of inertia, momentum, impulse, accuracy and
force reception
Newtons three laws of motion
Transfer of momentum and conservation of momentum
Leverage
Motion including human motion & projectile motion
Impact & friction
Balance & stability
Force production: any pushing or pulling activity that tends to alter the state of
motion of a body. Therefore, through application of a force, a body at rest can be
made to move, and a body in motion can be stopped, slowed, speed up or have the
direction altered.
Types of forces
Force without motion: isometric force
If the muscle length doesn’t change, then an isometric contraction or force is being
applied. Eg. pushing against an immovable object, or applying grip to a raquet or bat.
Force with motion: isotonic force
Force which changes state of motion of object eg. pushing out of blocks in 100m
sprint, shot putting, accelerating a hockey ball with push pass or decelerating a
football by marking it on your chest.
Submaximal force: Force is applied depending on what is required for activity eg.
putting a golf ball, drop shot in badminton. These sorts of skills require the performer
to use a limited number of muscle motor units in order to perform the skill or action
correctly (biomechanically sound) .
Maximal force: Perfect timing, maximum muscle contraction and excellent technique
achieve maximum force eg. high jump, serving in tennis, shot put. It is the result of a
combination of a number of forces.
Force summation: Can be achieved simultaneously – explosive action of all body
parts occurs at the same time eg. high jump take off.
Or sequentially – where the body parts move in sequence to generate force. This
technique used in gross body actions eg. throwing, kicking.
1. Use as many body parts as possible
2. Use largest body parts & muscle groups with greatest mass first eg. legs &
quads
3. Sequentially (in order) accelerate each body part so it momentum optimally
passes onto next body part
4. Sequentially stable each body part so the next body part accelerates around the
stable base (joint or part of body). Fast bowling in cricket is best example. In
the delivery action there is a sequence of body movements beginning with
larger parts (eg legs, trunk) and finishing with smaller, lighter body parts (eg.
wrist and hand). Ref page 54
Applying an effective force
Need to understand key words: inertia, momentum, impulse, accuracy & force
production
Inertia: body’s resistance to change its state of motion or objects resistance. The
heavy the object the greater the inertia, therefore greater force required to move it or
change its state of motion.
Momentum: Momentum = mass X velocity (speed)
Object can only have momentum if its moving. The greater the momentum the further
it may travel, and the harder it is to stop or slow also.
Impulse: = force X time
The longer a force can be applied, the greater a force applied, greater the objects
impulse or change of momentum will be.
Impulse & Accuracy
Different techniques in sport require longer contact with a ball when throwing or
hitting. The increase contact time provides greater force application and accuracy.
However, sometimes increased accuracy requires a decrease in force applied.
Force reception (absorption): Best for example when a ball is travelling at speed
towards catcher / fielder whoever or when a player is trying to crash, dodge through
opponents, the force (momentum) of the ball or player must be absorbed or received
over a distance to slow or stop it. For example, catching a ball in cricket, hands move
gradually back towards body as the fielder catches the ball.
Newton’s Laws of Motion
Newton’s first law of motion: inertia
An object whether at rest or in motion will continue in that state unless it is acted
upon by a force strong enough to change its state of motion or rest.
Newton’s second law of motion: acceleration / momentum
The acceleration of an object is directly proportionate to the amount of force applied
and it takes place in the direction in which the force is applied. For example a chest
pass in netball compared to the same chest pass but with a medicine ball. Obviously
heavier and slower too.
Newton’s third law of motion: action and reaction
For every action there is an equal and opposite reaction.
For example when a runner pushes against the ground exerting a downward force
against the ground (action force) but at the same time the ground exerts the same
force upwards and downwards in opposite direction against the runners foot (reaction
force).
Conservation of momentum (transfer of momentum)
Is that the total momentum of two objects before impact or contact will equal the total
momentum after impact. The momentum of impact is never lost, but rather transferred
on contact with other objects. Eg Billards when you hit a ball first, then next ball is hit
by that ball.
Levers: Levers are rigid bar like objects that turn about a fixed point called a fulcrum,
pivot or axis of rotation and to which forces are applied at two other points (effort
force and resistance or load force).
Types of levers
All levers have three main elements.
1. force arm
2. an axis, fulcrum or pivot point
3. a resistance arm
Two main functions of levers are to: increase or magnify force applied (occurs when
force arm is longer than resistance arm)
And to generate increased speed of movement (occurs when force arm is shorter then
resistance arm).
First class levers: axis is located between the resistance and point of force application
eg. see saw
Second class levers: are used to increase strength that humans apply to objects eg.
wheelbarrow the levers always have the resistance between axis and the force. The
force arm is always longer than the resistance arm.
Third class levers: mostly within the body, short force arm & long resistance arm for
speed advantage.
Levers in Sport (quite relevant)
People use a range of levers to increase the amount of momentum
Athletes increase length of external levers by using racquets, clubs, bats or even by
increasing levers with correct technique eg. over arm bowling in cricket, kicking a
footy.
Principal of leverage: is based on fact that velocity is greater at the end of long lever
than at the end of a short lever. The longer the lever the greater the velocity at impact
and the greater the momentum developed by the object.
Types of Motion
Linear motion: occurs when all the parts of an object travel over the same distance at
the same time, eg. ice skater gliding down the back straight after finishing a race.
Curvilinear motion: if a curved line is evident. Parabola: the flight path or
trajectory of a projectile (such as a ball, a javelin). This is an example of curvilinear
motion.
Angular motion: is evident when the body or an object turns about an axis of
rotation. For example, the shoulder joint in a throw, or the centre of gravity of an
object or human. In this case, the body parts closest to the axis of rotation move less
distance than do the body parts furthest from the axis of rotation. Check out the
diagram of a person able to rotate three axes on page 69 Live it Up text.
General motion: Is a known as a combination of both types of motion (angular &
linear). It can be described as linear motion of the whole body that is achieved by
angular motion of some parts of the body. For example, running a 100m sprint the
whole body moves in a straight line as a result of rotary motion of the legs about the
hip joint.
A rotational force is called an eccentric force. This occurs when the force is applied
away from the centre of gravity of an object. If only one eccentric force is applied,
then both linear & angular motion will occur. The amount of rotation produced by an
eccentric force depends on the magnitude of the force (greater the force applied, the
greater the angular velocity) & the distance between the point of force application
and the axis of rotation (moment arm). The product of this is called moment of force.
Moment of force = applied force X moment arm.
Angular momentum: depends not on mass and linear velocity but on the moment of
inertia & angular velocity. Angular momentum = moment of inertia X angular
velocity.
Conservation of angular momentum: is based upon Newton’s first law of motion.
As applied to angular motion this law states that ‘ a rotating body will continue to
rotate about its axis of rotation with constant angular momentum, unless acted upon
by an external force’. Given this, it follows that if angular momentum remains
constant, then moment of inertia and angular velocity are inversely proportional (as
one increases the other must decrease and vice versa).
Transfer of angular momentum: includes the notion that angular momentum can be
transferred from one body part to another. For example, when sitting on a swing a
person uses the action of the legs swinging forwards and backwards to get the swing
in motion. The angular momentum produced by the forwards and backwards
movement of the legs is transferred to the upper body causing the swing to move.
Projectile: is any object that is travelling through free air space. See page 78 for
varying examples. Factors affecting flight projectiles are speed of velocity of release,
angle of release, height of release, gravity, air resistance & spin.
Speed or velocity of release: is the single most important factor in achieving max
distance of a projectile. Greater the velocity of release the greater the distance
achieved.
Angle of release: this angle provides equal components of vertical and horizontal
force. However, this only applies when the height of release and the height of landing
are the same, and when spin and air resistance are not present. Optimal angle of
release is rare.
Height of release: this increases as the distance achieved increases. However, if the
height of landing is greater than the height of release, the horizontal range is
decreased.
Gravity: is the constant force which acts on all projectiles by pulling them towards
the earth.
Air resistance: effects the distance thrown by decreasing the distance of the throw or
hit.
Size or surface area of a projectile: the larger the surface area of a projectile, the
more it is affected by air resistance, reducing the surface area, particularly the frontal
surface area, is an important consideration in the design of objects and equipment that
act as projectiles in sport.
The nature of surface area of the projectile covering of a projectile affects the
degree of air resistance encountered. Rough textured covering, as opposed to smooth
surfaces increases the amount of air resistance experienced by the projectile during
flight.